iv is t-Drill Grinder. "IP^^^^^^WPHHWI 197 wheel, and fluting, &c., on the small wheel. It is, „,made, by Messrs. Selig, Sonnenthal & Co., and a small attachment is provided to carry the wheel when grinding milling cutters, the latter being then held on the spindle of the machine, and the wheel driven by catgut band Emery wheels may be used for general grinding and removing of surplus material and thereby save a large amount of fitting. (See p. 1030.) Twist-Drill Grinder.—These are of various designs,the one in Fig. 213 being made by Messrs. Selig, Sonnenthal & Co. The end of a twist-drill would be conical in shape but for the clearance or relief angle. The true surface becomes, accounting for clearance, a cone having a helix for its base, and enclosing an angle of. 118°. A section of this cone, then, made at right angles to one of the slant sides, would give a curve deviating slightly from a hyperbola, due to the clearance. We will now examine the method by which this hyperbolic surface is ground in Messrs, Selig's machine.